The art of click-type adjustable torque wrenches is old and highly developed. Such wrenches typically include elongate tubular lever arms with front and rear ends, manually engageable handles at the rear ends of the arms, and work engaging heads at the front ends of the arms. The work engaging heads have elongate polygonal parts with central axes normal to the longitudinal axes of the arms. The noted polygonal parts of the heads are provided to drivingly engage and carry drive sockets and the like to couple the wrench with a screw fastener or the like. The heads also have elongate rearwardly projecting levers that project freely rearwardly into and through the forward portions of the arms. The forward end portions of the levers are pivotally connected to the front end portions of the arms by pivot pins spaced rearward from and parallel with the axes of the heads and normal to the axes of the arms so that the levers can pivot laterally and into stopped engagement with the interior surfaces of the arms when the heads are drivingly coupled with a piece of work to be torqued and the arms are manually pivoted about the axes of the heads.
The rear ends of the levers have rearwardly disposed flat cam seats. The arms carry longitudinally shiftable plungers with flat forwardly disposed cam seats spaced from and opposing the cam seats on the levers. Cam blocks, with flat forwardly and rearwardly disposed cam faces are engaged between the noted cam seats and normally establish seated engagement therewith. Compression springs are engaged in the arms rearward of the plungers to engage and urge the plungers forward so that the cam faces of the cam blocks cooperate with their related cam seats to releasably hold the levers central and in axial alignment with the axes of the arms. Means are provided to adjust the axial biasing of and pressure exerted by the springs onto and through their related plungers, cam blocks and levers. The forces exerted by the springs determine the turning force which must be applied onto and through the wrench structures to cause the levers to overcome the holding force afforded by the cam blocks between the plungers and levers and to allow the levers to pivot within the arms. When sufficient force is directed through and between the arms and levers to cause the levers to pivot in the arms, the cam blocks are caused to pivot between the plungers and levers, the plungers shift rearwardly against the resistance of the springs and the levers pivot laterally and strike the inside surfaces of the arms, generating an audible click sound.
In the art of click-type adjustable torque wrenches of the general character referred to above, there is another more sophisticated and improved wrench structure which, in addition to the structure noted above, includes an elongate link with front and rear ends positioned in the wrench arm between the lever and the plunger. The front end of the link is pivotally connected with the rear end of the lever by a pivot pin on an axis parallel with and spaced rearward from the pivotal axis of the lever. The rear end of the link is formed with the rearwardly disposed cam seat which opposes and normally establishes flat engagement on the front cam surface of the cam block. The link is pivotally supported between its ends and within the arm by a fulcrum block which is carried by the link and engages the interior surface of the arm. When the arm and lever pivot, that is, when the rear end of the lever is caused to move laterally to one side or the other of the arm, the front end of the link moves laterally with it, pivoting the link about the turning axis of the fulcrum block. Upon such pivoting or turning of the link, the rear end thereof moves laterally to the other side of the arm. The rear end of the link cooperates with the cam block and plunger in the same manner that the levers, cam block and plungers of the first described form of torque wrench cooperate.
The fulcrum block in the second form of adjustable torque wrench is shiftable longitudinally on its related link and is releasably secured in stationary position therein by a set screw. The set screw is accessible through an opening in the side of the arm.
The above noted link affords a mechanical advantage for the spring of the wrench whereby the spring need not be overworked and whereby a lighter, more durable spring can be employed. Further, by adjusting the longitudinal position of the fulcrum on the link, accurate adjustments compensating for mechanical or structural deviations, which effect operation of the wrench can be made. For example, if the spring is too strong or too weak, the fulcrum block can be adjusted longitudinally of the link to compensate for the error in the strength of the spring.
The above and last noted wrench structure is the subject matter of and is fully disclosed in my U.S. Pat. No. 3,772,942 for "IMPROVED ADJUSTABLE TORQUE WRENCH" and issued Nov. 20, 1973. Wrenches of the type or class considered above are intended and designed to operate accurately in clockwise and counter-clockwise directions. In practice, however, such wrenches are seldom sym metrical, that is, they seldom work accurately in both directions. As a general rule, in prior art wrenches provided to operate through a range of from 0 to 150 ft. pounds of force, it can be anticipated that the operating force required to operate them in one direction is likely to be as much as 10 or 15 pounds different than the operating force required to operate them in the other or opposite direction. A 10 to 15 pound differential, while clearly unsatisfactory or excessive, has, as a practical manner, been considered acceptable since the prior art has failed to provide any means for correcting such deviations.
The noted deviations or errors between clockwise and counter-clockwise operation in such wrenches is often the result of the cumulative effect of the necessary tolerances in size and/or shape of parts and of necessary working tolerances between those parts. It has been determined that to establish prior art wrench structures so that they are symmetrical and operate accurately in both clockwise and counter-clockwise directions, extraordinary and excessively costly quality control procedures would have to be established and practiced and parts of the resulting wrench structures would have to be so closely fitted that normally encountered changes in temperatures and resulting thermal expansion and contraction of the parts would likely result in binding and/or freezing of the parts and render the wrenches inoperative.